The objectives of this project are to: (1)\identify the types and functions of mononuclear cells infiltrating the tumor mass at different stages of growth, i.e., from the time of tumor take through rapid growth and spontaneous regression; (2)\characterize the mechanism responsible for spontaneous regression of tumors; and (3)\determine the mechanism of maternal-offspring transfer of tumor immunity. During the past year, we obtained the following results: (1)\Competitive enzyme-linked immunosorbent assay (CELISA) technique was developed and used for quantitating serum tumor antigen. In a cross-sectional study of 25 tumor-bearing dogs with different tumor growth status, i.e., progressing, steady state, or regressing, we found that progressing tumors released greater amounts of tumor antigen per milliliter serum than either steady state (P less than 0.025) or regressing (P less than 0.001) tumors. In a time-course study of four tumor bearers, circulating tumor antigen was detected for 2 days following transplantation of 108 viable tumor cells, while persisting levels of tumor antigen became demonstrable 25 days post-transplantation. After surgical excision of the tumor, tumor antigen was cleared from the circulation within 72 hrs. Regression analysis of combined cross-sectional and time-course data showed that serum tumor antigen correlated positively with tumor volume and inversely related with the tumor leukocyte adherence (LAI) reactivity. (2)\Conglutinin-binding assay was adapted for the canine system to detect circulating immune complexes. The assay, which employs an enzyme-conjugated anti-dog IgG immunoglobulin to detect immune complexes bound to conglutinin, was capable of detecting as little as 5 micrograms of aggregated canine IgG per milliliter of serum within a maximum reaction time of 2.5 hrs. With this assay we were able to detect circulating immune complexes in 36% (23 of 64) of serum samples from tumor-bearing dogs. There was an inverse correlation between tumor volume and the immune complex levels in dogs with progressively growing tumors. (CS)